This invention is an outgrowth of U.S. Pat. No. 4,634,611 entitled "Flame Spray Method and Apparatus", issued Jan. 6, 1987.
In such apparatus, a flame sprayed coating is produced by combusting an oxy-fuel mixture in a duct to produce a high temperature, high velocity column, or flame jet. The jet is used to heat and propel a solid material from the duct toward the substrate. A stream of uncombusted compressed air, separate from the oxy-fuel mixture, is provided and heated above ambient temperature by flowing such a stream along the exterior of the duct while absorbing heat therefrom. The exit velocity of the flame jet is maintained subsonic while surrounding the column or flame jet with a co-axial sheath of expanded compressed air having a velocity which is sufficiently close to the velocity of the flame jet so that there is little initial mixing of the sheath and the jet.
While such apparatus and method, as set forth in U.S. Pat. No. 4,634,611, is an improvement within the developing art of flame spray of coatings for impact and solidification on a substrate, the cooling effect by the air prior to impact is less than satisfactory.
It is therefore a primary object of the present invention to produce a hot jet surrounded by a cooling sheath of a high velocity gas surrounding the hot jet with an enveloping flow of steam and effecting significantly increased cooling by evaporation of water passed over the hot surfaces of the combustion nozzle to produce the steam as an annular sheath about the flame spray jet.